PURPOSE: To prevent the generation of a dangerous blowout by sandblasting the pressing faces between the piston and cylinder constituting a mold in the extra-high pressure and high temp. generator to improve the surface property.

CONSTITUTION: In the extra-high pressure and high temp. generator for producing an extra-high pressure sintered compact of diamond, etc., a mold is formed with a couple of piston 1 and cylinder 2, the outer laminated-paper gasket of a gasket 7 is inserted into the pressing part between the piston 1 and cylinder 2, hence the gasket is appropriately fluidized when pressure is applied to prevent the pressure decrease in a sample part, and a high pressure is formed. In this case, the pressing faces of the piston 1 and cylinder 2 in contact with the gasket are ground, then sandblasted and mat-finished to the surface roughness Rmax of 4-15μm. The friction coefficient between the gasket and the piston and cylinder faces is increased, a sealing force against the force to push the gasket outward is increased, and the generation of blowout is prevented.